1. Field of the Invention
This invention relates to the lamination of rigid transparent sheets of glass or glass substitutes, such as polycarbonates, polyester resins, acrylic plastics and the like, with flexible interlayer materials such as polyvinyl butyral, polyurethanes and the like. The lamination of such materials produces laminated safety glass which has been developed for various uses, especially as windshield closures.
Flat assembles to be laminated have been initially treated by a prepressing operation to bond the plastic interlayer to the opposing surfaces of the relatively rigid layers by heating the assembly and then passing the heated assembly between a pair of resilient nipper rolls. A typical glass rolling apparatus containing conventional resilient nipper rolls used in commercial prepressing operations is disclosed in U.S. Pat. No. 2,673,168 to Pascoe and Rugg. Another technique employed to prepress such assemblies to be laminated is disclosed in U.S. Pat. No. 2,948,645 to Keim. In this latter patent, the periphery and the margin only of the assembly is enclosed in a flexible channel shaped member made of a fluid impervious material to place the member in engagement with a marginal portion of the outer surfaces of the assembly and in spaced relation to at least part of the periphery of the assembly to provide a conduit adjacent the periphery of the interfaces between the interlayer and the relatively rigid glass sheets, air is evacuated from the conduit through the member to remove air from between the interlayer and the glass sheets, the assembly and the material enclosing it are heated to an elevated temperature to bond the interlayer to each glass sheet in at least its marginal area while continuing the evacuation to maintain the vacuum in the conduit, the member enclosing the assembly is removed, and the assembly is ready for a final pressing operation.
Regardless of whether the prepressing step is performed by the roll pressing apparatus depicted in the Pascoe et al patent or by the peripheral evacuation technique as disclosed in the Keim patent, the prepressed assembly is then ready for final lamination where the assembly is directly exposed in an autoclave to an elevated temperature and an elevated fluid pressure substantially above atmospheric pressure and sufficient to bond the interlayer to the glass sheets substantially throughout the entire area of the assembly. As long as the relatively rigid sheets of glass or glass substitutes are very smooth, the prepressing operation, whether performed by nipper rolls or by peripheral evacuation channels, is sufficient to prepare the assembly for its final laminating step in an oil autoclave. However, when the surface of the relatively rigid sheet of glass or glass substitute material is not smooth and has either a wavy appearance or is provided with departures from smoothness caused by engaging the glass while hot with solid materials or glass handling elements needed to support the rigid material during a thermal treatment such as thermal tempering prior to assembling the elements for lamination, the prior art technique for prepressing has been found to be unsuitable. Oil has penetrated the prepressed assembly during the final laminating step in an oil autoclave.
In the past, oil penetration has been avoided by assembling the layers of rigid transparent material and interlayer material to form an assembly to be laminated. The assembled layers were taped together and the taped assembly inserted within a flexible bag which was then sealed and evacuated. The sealed bag and its contents were inserted in an oil autoclave and subjected to elevated heat and pressure for sufficient time to complete the lamination and produce a transparent laminated assembly. After lamination, the bag was opened and the laminated transparent assembly removed. Such a technique uses a flexible bag of the type described in U.S. Pat. No. 3,255,567 to Keslar and Rankin. While such bags avoided oil penetration during lamination, the cost of the bags and of the labor needed to perform a laminating operation using such bags made it economically unfeasible to use such a laminating method except in cases where the cost of the ultimate laminated window was not of any concern.